Lactate dehydrogenase
Enzyme Structure Lactate dehydrogenase is a tetramer (Figure 1) consists of either LDH muscle type (M) or heart type (H) subunits and each subunit has a molecular weight of approximately 35 kDa (1). Each monomer subunit has a total of 330 amino acid residues. It is classified as mixed beta-alpha-beta, with mainly parallel beta sheets. The binding pocket of the enzyme consists of an overall alpha/beta structure which is highly conserved and consists of residues 163-247 and 267-331. The lactate-NAD+ complex bind to the active site which is located in the binding pocket and contains the catalytic His 193, Asp 168, Arg 171, Thr 246 and Arg 106 (2). Normal Function The Lactate Dehydrogenase (LDH) is a key enzyme in the process of glycolysis. Its activity occurs at the end of the glycolysis when pyruvate is produced. It is responsible for recycling NAD+ by interconvert between pyruvate and lactic acid using the proton from NADH (3). The reaction and mechanism catalyzed by lactate dehydrogenase is shown below (Figure 2) (4): Without lactate dehydrogenase, lactate is not able to converted back into pyruvate and enters into the kreb cycle for energy production during aerobic condition. Therefore, the patient would experience exercise intolerance which includes symptoms of fatigue, muscle pain, and cramps during exercise (5). Observations in Alzheimer's Disease In M. Bigl et al’s experiment, the tissues of the AD patients were obtained using autopy. (CC) Cerebellar cotex, (BF) basal forebrain, (HC) hippocampus, (ST) striatum, (FC) frontal cortex and (OC) occipital cortex tissue samples from the AD patients and healthy individual brain were ultrasonificated and the supernatant of the centrifuged samples were obtained for LDH activity assay (6). According to M. Bigl et al., the activity of LDH were found significantly higher in the frontal cortex and basal forebrain in the Alzheimer patients when compared to the healthy control (Figure 3). However, the increase in activity of LDH was more significant. It was confirmed by using western blotting which the amount of LDH showed a significant increase in the frontal cortex of the patients (Figure 4) (6). This increase in LDH can be explained using the astrocyte- neuron lactate shuttle model (ANLS) (Figure 5). This model suggests that astrocytes are able to take up glucose from cerebral blood vessels through GLUT or by breaking down glycogen in the astrocytes. This glucose is then converted into lactate through glycolysis and released into the extracellular space via monocarboxylate transporters (MCTs). Extracellular lactate is then taken up by neurons and converted to pyruvate by lactate dehydrogenase (LDH) (7). Since there is a reduction of glucose metabolism in the brain for the AD patients, the increase in LDH in the patients is probably due to the increase in astrocyte LDH activity which provides lactate to the neurons for energy production. ---- Back to homepage. Works Cited 1. Labrou, N.C., YD. (1995) Biomimetic dye affinity chromatography for the purification of bovine heart lactate dehydrogenase. J Chromatogr A. 718, 35-44 2. Read, JA. Winter, VJ. Eszes, CM. Sessions, RB. Brady, RL. (2001) Structural basis for altered activity of M- and H-isozyme forms of human lactate dehydrogenase. Proteins. 43, 178-185 3. Markert, CL. (1984) Lactate dehydrogenase. Biochemistry and function of lactate dehydrogenase. Cell Biochem Funct. 2(3), 131-134. 4. Hou, R. Chen, Z. Yi, X. Bian,J. Xu, G. (2000) Catalytic reaction mechanism of L-lactate dehydrogenase: an ab initio study. SCIENCE IN CHINA. 43, 587-599 5. Miyajima, H. Takahashi, Y. Kaneko, E. (1995) Characterization of the glycolysis in lactate dehydrogenase-A deficiency. Muscle Nerve. 18,874-878 6. Bigl, M.; Bruckner, M. K.; Arendt, T.; Bigl, V.; Eschrich, K. Activities of key glycolytic enzymes in the brains of patients with Alzheimer's disease. J Neural Transm 1999, 106, 499-511. 7. Newington, JT. Harris, RA. Cumming, RC. (2013) Reevaluating Metabolism in Alzheimer's Disease from the Perspective of the Astrocyte-Neuron Lactate Shuttle Model. Journal of Neurodegenerative Diseases. 2013, 1-13